1. Field of the Invention
The present invention relates to barium titanate semiconductor ceramics. More particularly, the present invention relates to a barium titanate semiconductor ceramic having a positive resistance-temperature coefficient and a semiconductor ceramic device using the same.
2. Description of the Related Art
Barium titanate semiconductor ceramics having positive resistance-temperature characteristics (PTC characteristics), in which resistivity at room temperature is low and resistance rapidly increases above a certain temperature (the Curie temperature), have been widely used in temperature control, current control, heating at constant temperature, and the like. Among these applications, an overcurrent protection device used for circuits having lower resistivity while being compact and having a high withstand voltage has been desired.
A conventional technique relating to the present invention is disclosed in Japanese Unexamined Patent Application Publication No. 8-217536. The conventional technique focuses on the sodium content contained in a barium titanate semiconductor ceramic and discloses that resistivity of a barium titanate semiconductor ceramic can be adjusted by adding 0.0005 to 0.02 percent by weight of sodium thereto. According to this conventional technique, when resistivity of a baked material changes due to the variation in baking temperature therefor, the resistivity of a finished semiconductor ceramic composition is adjusted by controlling the sodium content to range from 0.0005 to 0.02 percent by weight.
In addition, the publication discloses that the withstand voltage is decreased by adding sodium in an amount of 0.03 percent by weight or more.
The publication describing the conventional technique makes no reference to particle diameters of crystals contained in a semiconductor ceramic. However, when the desires for lower resistivity and a higher withstand voltage are focused on, the inventors of the present invention found that the resistivity and withstand voltage desired cannot always be obtained only by controlling the sodium content as described above.
Accordingly, it is an object of the present invention to provide a semiconductor ceramic having a positive resistance-temperature coefficient in addition to having lower resistivity and a higher withstand voltage, and to provide a semiconductor ceramic device using the semiconductor ceramic mentioned above.
The semiconductor ceramic of the present invention has a positive resistance-temperature coefficient and comprises barium titanate as a major component and sodium. In order to solve the technical problems described above, the average particle diameter of the semiconductor ceramic is about 7 to 12 xcexcm and the sodium content is about 70 parts per million (hereinafter referred to as ppm) or less on a weight basis.
In addition, the present invention can be applied to a semiconductor ceramic device comprising a body composed of the semiconductor ceramic described above and electrodes disposed on the body.